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PHOSPHATE CHEMISTRY 

AS IT CONCERNS THE MINER. 

» 

BY 


THOMAS M. CHATARD, 

WASHINGTON, T>. C. 


A Paper read before the American Institute of Mining Engineers, 

Baltimore Meeting, February, 1892 . 



AUTHOR’S EDITION. 
1892 . 





























PHOSPHATE CHEMISTRY AS IT CONCERNS THE MINER. 


BY THOMAS M., CHATARD, WASHINGTON, I). C. 

Every one engaged in the mining of phosphates is well aware 
that the price he gets for his product depends upon the results of 
chemical analysis. He knows that the value rises with the percent¬ 
age of phosphoric acid, whether stated as such or as so-called “ bone 
phosphate,” and that deductions are made for certain constituents, 
such as alumina or oxide of iron, when the amount of these exceeds 
certain limits. The reason for these deductions is but vaguely un¬ 
derstood by many, and there are few who possess the special knowl¬ 
edge required to form a competent judgment, either as to the reli¬ 
ability of the analysis on which so much depends, or as to how far 
and in what manner they may avail themselves of chemical inves¬ 
tigation to aid them in improving the quality and quantity of their 
output. 

The acquisition of this special knowledge is by no means easy, 
especially for the busy man who must pick it up as best he may and 
at odd times. So far as reading can aid, there are two books readily 
accessible. One is The Nature and Origin of Deposits of Phosphate 
of Lime , by E. A. F. Penrose, Jr., being Bulletin No. 46 of the 
United States Geological Survey, which, in addition to its own valu¬ 
able matter, contains an extensive list of other publications on the 
subject. The other is the recently published Phosphates of America, 
by Dr. Francis Wyatt, which gives much information concerning the 
mining, preparation, manufacture, and analysis of phosphates. The 
remaining phosphate literature, though very extensive and rapidly 
increasing, is rarely in such shape as to be readily available to any 
one but the specialist. This is particularly the case with the descrip¬ 
tions of analytical methods, which are often vague even to the prac¬ 
ticed chemist; and the proofs of the value of which are, in many 
cases, quite inadequate, while the vigor displayed in the attack and 
defence of views and methods is apt to contuse the non-professional 
reader. Nevertheless, whatever knowledge can be obtained from 



2 PHOSPHATE CHEMISTRY AS IT CONCERNS THE MINER. 


books will be found of value when the adviee and assistance of the 
chemist is sought. 

There are chemists and chemists; there are experts and pre¬ 
tenders; and if one has no previous knowledge of the subject, how 
can one distinguish between the two, and how can one weigh the 
value of advice or work. The greater the knowledge of the miners, 
the more frequent and exacting will be their demands on the chem¬ 
ists, who, in turn, will be stimulated and encouraged to make them¬ 
selves—not what too many of them are, irfere analysts, but what they 
ought to be—chemical engineers, having a practical knowledge and 
a clear comprehension of the principles and details of technical 
operations in the lines in which they profess to work. One hears 
much complaint among chemists that practical men are unwilling to 
pay a fair price for good work, and do not appreciate it when they 
get it. While this is unfortunately true in many instances, I cannot 
but think that it is, to a considerable extent, the fault of the chemists 
themselves. When called upon for advice as to the proper system 
and appliances for carrying out a given piece of practical work, how 
many of them are really capable of giving a reliable opinion based 
upon facts and tigures of experience? They may be able scientific 
men; they may be accomplished analysts; but of industrial opera¬ 
tions they have but the vaguest impressions, on which, too often, 
very extensive assertions are based. It cannot be too strongly in¬ 
sisted upon that those who desire to aid in the development of chem¬ 
ical industry should make themselves chemical engineers. There is 
the science of electricity; there is the profession of the electrical 
engineer ; and it is hardly necessary to say that a man may be deeply 
learned in the sciences of geology, chemistry, mechanics, and heat, 
and yet have no claim to be considered a mining engineer. The 
chemical engineer, as distinguished from the mere analyst, must be 
able and ready not only to answer, through knowledge and investi¬ 
gation, the questions of his clients, but also to indicate to them new 
lines of progress. Such a man educates his clients; they, in turn, 
develop him; and only through such beneficial mutual reaction can 
we hope to reach that much to be desired time when, through a 
proper concert of action among miners and shippers of' phosphate, 
as urged by Dr. Wyatt, uniformity in analytical methods and prac¬ 
tice shall be enforced. 

The present frequent occurrence of widely differing results be¬ 
tween chemists working upon the same sample, can then, in great 
measure, be avoided, and the consequent vexations and expensive 


PHOSPHATE CHEMISTRY AS IT CONCERNS THE MINER. 


3 


disputes and lawsuits be prevented. The eighth chapter of Dr. 
Wyatt’s book shows the advantages to be gained by such uniformity, 
and if all who are interested in establishing it will do their part, the 
arrival of the chemical millenium can be much hastened. 

One of the obstacles is that there are different ways of determin¬ 
ing each constituent of a phosphate. Each method has its defenders 
and opponents, and each chemist, if free to do so, will prefer to use 
his favorites, which, in his hands at least, yield results which satisfy 
him. Now, every one can readily understand that if two chemists, 
analyzing the same sample, use different methods, the results may, 
and probably will, vary widely. It is not so well known that, even 
when they use the same general methods, the results may differ de¬ 
cidedly if there are variations in the practical details of manipula¬ 
tion. The operative detail is very often the essence of a good method, 
and should, when published, be described at length, especially when 
intended for commercial analysis. No method should be adopted for 
the valuation of materials which has not already met with general 
favor after undergoing; a searching; ordeal of test and discussion ; and 
a method once adopted should be carried out to the letter. Under 
such a system the honest, conscientious, painstaking worker will be 
appreciated. 

The methods which have been used for the chemical work of the 
United States Geological Survey in Florida will be fully described 
at the end of this paper, so that they may be easily followed by others, 
and possible errors maybe detected. None of them are original. I 
have preferred, for the reasons given above, to use such as have already 
attained a wide acceptance among chemists. Nor have any very im ¬ 
portant modifications been introduced, except in the determination 
of fluorine. Tf they are found to differ greatly from those advo¬ 
cated by some other chemists, they are offered in no spirit of conten¬ 
tion or criticism, though it may be necessary, here and there, to point 
out these differences and their reasons. What we need are accurate 
standards, carefully described ; and such I have tried to furnish. 
When the miner can see for himself what skill and labor are in¬ 
volved in the making of a trustworthy analysis, he ought to be 
willing to pay a fair price for good work. 

The commercial valuation of a natural phosphate is based on an 
analysis showing a number of constituents, only one of which, the 
phosphoric acid, has any value. All the rest are either useless (like 
water and insoluble matter, which merely reduce the percentage of 
phosphoric acid) or hurtful (like carbonates, which, neutralizing 


4 PHOSPHATE CHEMISTRY AS IT CONCERNS THE MINER. 


their equivalent of sulphuric acid, increase the cost of manufacture; 
or, like alumina and ferric oxide, which not only “ revert ” a portion 
of the u soluble ” phosphoric acid, but also, in proportion to their 
amount, tend to render the superphosphate wet and unmanageable). 
Moreover, the amount of fluorine may be of great importance, since 
there are silicates of alumina which, otherwise unattacked by the 
sulphuric acid, are decomposed in the presence of fluorine, the other¬ 
wise inactive alumina assuming an obnoxious form. The fluorine, 
being apparently a constituent of the phosphate, cannot be removed 
by the miner, but, as pointed out by Dr. Wyatt, the greater part of 
the sand and clay can be got rid of by careful and systematic wash¬ 
ing; and he also shows effectively the need and advantages of a 
proper chemical control of mining operations. The following fig¬ 
ures, though but the results of laboratory experiments, may be of 
interest in this connection : 

To observe the effect of a dry concentration, some “ rock-phos¬ 
phate ” (No. 74, Florida collection) was crushed and passed through 
a 10-mesh sieve. The material was then sifted on a 20-mesh sieve, 
yielding A (coarser than 20-mesh) 64.62 per cent., B (finer) 35.38 
per cent. B 2 was washed in a sort of spitzkasten , and gave: 

Per cent. Per cent. 

Bj (coarsest), .... 34.10 or 12.06 of original material. 

B 2 (middle), .... 15.00 “ 5.31 “ “ . 

B 3 (finest), . . . . 46.10 “ 16.31 “ “ 

Loss in suspended matter, . 4.80 “ 1.70 “ <l 

In each portion the insoluble matter, phosphoric acid and the 
combined weight of the alumina and ferric oxide (equal to half the 
weight of the precipitated phosphates) were determined as follows, 
the phosphoric acid being given as such, and also calculated into its 
equivalent of calcium phosphate : 



Total sample. A 

B 

B, 

b 2 

B 3 

Insoluble, 

. 5.85 .53 

8.56 

.51 

6.26 

14.53 

A1 2 F e 2 0 3 , , 

. 5.13 3.36 

6.32 

2.41 

3.54 

7.79 


. 33.98 37.83 

32.60 

37.44 

34.69 

29.98 

Ca 3 P 2 0 8 , . 

. 74.20 82.38 

71.18 

81.70 

75.75 

65.63 


Judging from the percentage of Al 2 , Fe 2 , O a , we find that, by a 
simple mechanical process we have obtained in A, B t , and B 2 , high- 
grade phosphate amounting to 82 per cent, of the original sample, 
while B 3 contains too much of these oxides to be readily salable. 

A more difficult problem is illustrated by the results of a similar 


PHOSPHATE CHEMISTRY AS IT CONCERNS THE MINER. 5 


treatment of some of the “ phosphatic sand-rock ” of the Itche- 
tucknee region in Florida. This is of low-grade in phosphoric acid, 
and contains much fine sand and clay; and a test was made to see 
it these impurities could be removed by the same operation as before. 
The sample yielded, in percentage, A, 20.74; B, 79.26; and B con¬ 
sisted of B n 3/.83; B 2 , 20.87; B 3 , 17.75, and loss, 2.81. Analysis 
gave: 


Insoluble, 
Al 2 Fe 2 0 3 , 
PA, . 
Ca 3 P 2 08, . 


Total sample. A B B t B 2 B 3 

. 34.77 18.13 44.19 62.55 38.11 10.50 

. 7.09 6.53 7.70 2.96 7.97 15.57 

. 24.08 30.49 20.36 13.75 25.02 31.90 

. 52.49 65.46 43.38 29.97 54.54 69.54 


Evidently, we have here a substance very different from the pre¬ 
ceding one. The first is a metamorphosed rock; the second, the 
product of a sedimentation, by which the phosphate, sand and clay 
have been so intimately mixed that it will be difficult to devise a 
concentration-method which will furnish a phosphate suitable for the 
fertilizer-trade. 

A third example is the “ land-pebble,” with its matrix, from the 
vicinity of Bartow, Florida. A sample disintegrated with water, 
without crushing, and thorougly washed in a 20-mesh sieve, gave 
“ pebble ” 54.54, “ matrix ” 45.46 per cent. The analytical results 
of present interest are : 


Insoluble, 
Al2Fe 2 0 3 , . 
P 2 0 5 , . 

Ca 3 P 2 0 8 , . 


Pebble. 

Matrix. 

. 4.34 

49.78 

. 4.05 

9.38 

. 34.72 

13.58 

. 75.69 

29.60 


These figures show that simple washing on sieves, down to 20- 
mesh, is the best way to handle this material. Possibly, by washing 
the matrix further, on a 40-mesh sieve, a coarse product of some 
value might have been obtained ; but the practical limit has probably 
been reached as above. 

It would be easy for any one desiring to make such investigations 
to construct simple washing apparatus capable of treating large sam¬ 
ples. The different grades of product should be dried and sampled, 
and the samples should be sent to a chemist for analysis. In many 
attempts to devise some simple apparatus for approximate determi¬ 
nation of the phosphoric acid, at least, without special laboratory 
facilities, I have met with no success. The measurement of the 
volume of the yellow molybdate precipitate has been often proposed 



6 PHOSPHATE CHEMISTRY AS IT CONCERNS THE MTNER. 


as a means of rapid estimation of phosphoric acid; but numerous 
series of experiments, by myself and others, have shown that the 
information thus gained is but little more accurate than the estimate 
formed upon inspection by the practiced eye of the miner. 

The Analysis of a Natural Phosphate. 

f , t 

The sample should be ground fine enough to leave no residue on 
an 80-mesh sieve, and should be thoroughly mixed by passing it 
three times through a 40-mesh sieve. 

Moisture. —Two grammes are weighed into a tared platinum cru¬ 
cible. This, with its lid, is placed in an air-bath at J05° C., and 
heated for at least three hours. The lid is then put on, and the cru¬ 
cible is placed in a desiccator and weighed as soon as cold. The loss 
in weight is the moisture. 

Combined Water and Organic Matter. —The residue from the 
moisture-determination is gradually heated to full redness over a 
Bunsen lamp, and then ignited over the blast-lamp. This operation 
is repeated alter weighing until a constant weight is obtained. The 
loss (after deducting the percentage of carbonic acid as found in 
another portion) may be taken as water and organic matter. This 
method is sufficient for all practical purposes; but when minerals 
containing fiuorine are strongly ignited, a part of the fluorine is ex¬ 
pelled ; hence, if more accurate determinations are required, the 
methods given in the standard works on analysis may be used. 

Carbonic Acid. —Many forms of compact apparatus have been 
devised for this estimation, but none of them are satisfactory if accu¬ 
rate results are desired. Not to mention other objections, many phos¬ 
phates must be heated nearly to boiling-point with dilute acid to 
effect complete decomposition of the carbonates. The distillation- 
method, described by Gooch (Bulletin No. 47, United States Geo¬ 
logical Survey), is excellent, and when once the apparatus is set up, 
its work will be found to be rapid and satisfactory. 

Insoluble Matter , Phosphoric Acid, Alumina , Ferric Oxide, Lime , 
and Magnesia. —Five grammes of the phosphate are put into a 
beaker; 25 c.c. nitric acid (specific gravity 1.20) and 12.5 c.c. hy¬ 
drochloric acid (specific gravity 1.12) are added; and the beaker, 
covered with a watch glass, is placed upon the water-bath for thirty 
minutes. The contents of the beaker are well stirred from time to 
time, and at the end of the period the beaker is removed from the 
bath, filled with cold water, well stirred, and allowed to settle. The 


PHOSPHATE CHEMISTRY AS IT CONCERNS THE MINER. 7 


solution is next filtered into a 500 c.c. flask, and the residue is thor¬ 
oughly washed with cold water, partially dried, and then ignited 
(finishing with the blast-lamp) and brought to constant weight. The 
figures thus obtained will, however, be incorrect, because the fluorine 
liberated during the solution of the phosphate dissolves a portion of 
the silica. Hence, the results are too low. Nevertheless, as the 
same reaction would occur in the manufacture of a superphosphate 
from the material, the determination may be considered as a fair 
approximation to commercial practice. The ignited residue must be 
tested for P 2 0 5 . 

The flask containing the filtrate is filled up to the mark with cold 
water, and the solution is thoroughly mixed by twice pouring into a 
dry beaker and returning it to the flask. Cold water—not hot water, 
as recommended by Dr. Wyatt—is used for washing the residue, 
since if hot water is used, the sesquichlorides are apt to become basic 
and insoluble, and hence to remain in the residue and filter-paper. 
Besides, as the flask is to be filled to the mark, the contents must be 
cold before any volumetric measurements can be made. 

Phosphoric Acid .—Two portions of the solution, 50 c.c. each = 
.5000 gramme of original material, are put into beakers and evapor¬ 
ated until all the hydrochloric acid is driven off. (Although it is 
asserted that a moderate amount of this acid does not interfere with 
this determination, I prefer to be sure of its absence.) Then 150 
c.c. of molybdate solution are added to each portion, which is well 
stirred and allowed to stand on the water-bath until quite hot; then 
removed and allowed to stand until perfectly cold. It is best to let 
it stand for at least three hours, after which the yellow precipitate is 
filtered and well washed with a 20 per cent, solution of ammonium 
nitrate, containing of its volume of nitric acid (which I prefer to 
use, although the precipitate is considered, on good authority, to be 
insoluble in cold water). The filtrate should be tested for any re¬ 
maining P 2 0 5 by adding some molybdate solution and digesting it 
for some time. The funnel, with its contents, is now inclined over 
the beaker in which the precipitation was effected, and the precipitate 
is washed back into it with a jet of pure water. Ammonia water 
in slight excess is then added, and on gently warming the beaker 
complete solution should take place. Any residue indicates either 
incomplete washing or, under some circumstances, silica. The solu¬ 
tion is filtered through the same filter into a clean beaker, and the 
first beaker and the filter are washed with ammonia wash-water (1 
part strong ammonia to 4 parts water). The filtrate is now brought 


8 PHOSPHATE CHEMISTRY AS IT CONCERNS THE MINER. 


to a boil and removed from the lamp, and magnesia solution is 
added, drop by drop, with continual stirring. The precipitate at first 
redissolves, but during the continued addition of the magnesia the 
solution becomes cloudy with a flocculent precipitate, which, how¬ 
ever, as the stirring is continued, becomes crystalline and subsides 
rapidly. When further addition of the precipitant causes no cloudi¬ 
ness, and the crystalline change is complete (which is most im¬ 
portant), the beaker is placed in very cold water to chill its contents 
as rapidly as possible. When perfectly cold, it is again tested with 
a drop of magnesia solution, and, if the precipitation is found to be 
complete, about one-third of its volume of strong ammonia is added, 
the whole stirred and allowed to stand at least three hours (although 
I have had perfectly good results after the lapse of not more than 
one hour). 

The precipitate is finally filtered on an asbestos felt* in a Gooch 
perforated crucible, and washed with the 1 : 3 ammonia-water. As 
the removal of the precipitate from the sides of the beaker requires 
much rubbing and much wash-water, the washing will be complete 
as soon as the beaker is clean. Two or three drops of a strong solu¬ 
tion of ammonium nitrate are poured on the precipitate, which is 
then carefully dried and gently heated until the fumes of ammonium 
salts ceases to come off. The heat is then increased, and as soon as 
the glow of the pyrophosphate formation has passed through the 
whole of the precipitate, the crucible is placed in a desiccator and, 
when cold, weighed. The ignited precipitate is very white, and the 
difference between the two determinations of P 2 O s ought not to ex¬ 
ceed .05 per cent, for thoroughly satisfactory work, although, as 
shown in the column of analytical results at the end of this paper, 
it sometimes exceeds this limit, since, as but one half-gramme is 
used for each determination, any error is doubled in the final calcu¬ 
lation. 

In default of a Gooch crucible, the ammonium-magnesium-phos¬ 
phate should be filtered on paper, and, after washing, dissolved in 
dilute nitric acid, evaporated in a platinum crucible to complete 
dryness, carefully ignited and weighed. A clean mass is thus ob¬ 
tained, while, on the other hand, if the precipitate be ignited with 
the paper, it is difficult to destroy the carbon. 

Lime, Alumina and Ferric Oxide .—The Glaser method for alumina 
and ferric oxide was used in the earlier part of the work here de- 


* Gooch, Am. Chem. Jour., vol. i., p. 317» 





PHOSPHATE CHEMISTRY AS IT CONCERNS THE MINER. 9 


scribed, but later the method as improved by R. Jones ( Zeitschriftf. 
Angew. Chemie , 1891, Heft I.) was adopted and has been found en¬ 
tirely satisfactory. As I use it, the process is as follows: 

Lime .—One hundred centimeters of the solution (containing 1 
gramme of the original substance) are evaporated in a beaker to 
about 50 c.c.; 10 c.c. of dilute sulphuric acid (1 c.c. H 2 S0 4 diluted 
to 5) are added; and the evaporation is continued on the water-bath 
until a considerable crop of crystals of gypsum has formed. The 
solution is then allowed to cool, when it generally becomes pasty, 
owing to the separation of additional gypsum. When it is cold, 150 
c.c. of 95 per cent, alcohol are slowly added, with continual stirring, 
and the whole is allowed to stand for three hours, being; stirred from 
time to time. After three hours it is filtered, with the aid of a filter- 
pump, into a distillation-flask, and the beautifully crystalline pre¬ 
cipitate, which does not adhere to the beaker, is washed with 95 per 
cent, alcohol. The filter, with the precipitate, is gently removed 
from the funnel and inverted into a platinum crucible, so that, by 
squeezing the point of the filter, the precipitate is made to fall into 
the crucible, and the paper can be pressed down smoothly upon it. 
On gentle heating of the crucible, the remaining alcohol burns off, 
and when the paper has been completely destroyed, the heat is raised 
to the full power of a Bunsen lamp for about five minutes, after 
which the crucible can be cooled and weighed. The precipitate is 
then CaS0 4 , and the results are very close, a pair of determinations 
of CaO (calculated from the CaS0 4 ) rarely differing more than .05 
per cent., as will be seen below. 

Alumina and Ferric Oxide .—The distillation-flask containing the 
alcoholic filtrate is connected with its condenser and heated on a 
water-bath until no more alcohol comes over. This distillate, if 
mixed with a little sodium carbonate and redistilled over quicklime, 
can be used over and over again, so that the expense for alcohol is 
really very slight, while in the use of the Glaser method, with its 
large amount of sulphuric acid, all the alcohol is lost. 

When the distillation is ended, the residue in the flask is washed 
into a small platinum dish and evaporated as far as possible on the 
water-bath. It will become dark-brown, owing to the presence of 
organic matter, which must be destroyed, since it prevents the com¬ 
plete precipitation of the phosphate in the subsequent operation. 
This destruction of the organic matter is best affected, according to 
my experience, by removing the dish from the bath, adding a small 
quantity of pure sodium nitrate and heating very carefully over the 


10 PHOSPHATE CHEMISTRY AS IT CONCERNS THE MINER. 


naked flame, while keeping the dish well covered with a watch-glass. 
If care he taken, there will he no loss hy spattering; and the mass 
fuses to a colorless, viscous liquid, cooling to a glass, which is readily 
soluble in hot water made acid with nitric acid. 

The solution is transferred to a beaker, made slightly (but dis¬ 
tinctly) alkaline with ammonia; then carefully neutralized with 
acetic acid ; then diluted with hot water, brought to the boil, allowed 
to settle and filtered. After the precipitate, which does not adhere 
to the beaker, has been completely brought upon the filter by means 
of hot water, the washing is completed with a solution of ammonium 
nitrate (made by neutralizing 5 c.c. HN0 3 with ammonia and dilut¬ 
ing to 250 c.c.), and the precipitate is dried, ignited at full lamp- 
heat, cooled and weighed. As the determinations are made in pairs, 
one portion is used for the estimation of the phosphoric acid by fusing 
it with a little sodium carbonate, dissolving in dilute nitric acid and 
treating with molybdate solution as already described, while the 
other portion, also fused with sodium carbonate, is dissolved with 
sulphuric acid, and the iron is reduced and titrated with permanga¬ 
nate. 

The average of the results by this method will show that the 
weights of the two portions of the aluminum ferric phosphate (usu¬ 
ally symbolized alfe) should not differ more than 1 milligramme, 
which, upon the conventional division by 2, gives a difference of 
.05 per cent, for the combined oxides; but, in general work, if the 
results for the combined oxides do not vary more than one-tenth of 
1 per cent., the method ought to be considered very satisfactory, since 
it is easy to execute, and requires but little time in actual labor. 

Magnesia . 1 — 1 The filtrate from the alfe is evaporated to a small 
bulk, made strongly ammoniacal, and allowed to stand ; when mag¬ 
nesia, if present, will separate as the double salt, and should be 
treated as usual. If, during the evaporation of the filtrate (which 
should be perfectly clear at first) any flocculent matter separates, this 
should be filtered off 1 and examined before precipitating the mag¬ 
nesia. 

Fluorine .—Two grammes of the phosphate are intimately mixed 
in a large platinum crucible with 3 grammes of precipitated silica 
and 12 grammes of pure sodium carbonate, and the mixture is gradu¬ 
ally brought to clear fusion over the blast-lamp. When the fusion 
is complete, the melt is spread over the walls of the crucible, which 
is then rapidly cooled (preferably by the blast of air). If this has been 
properly done, the mass separates easily from the crucible, and the 


PHOSPHATE CHEMISTRY AS IT CONCERNS THE MINER. 11 


subsequent leaching; is hastened. The mass, detached from the cru¬ 
cible, is put into a platinum dish, into which whatever remains ad¬ 
hering to the crucible, or its lid, is also washed with hot water. A 
reasonable amount of hot water is now put into the dish, which is 
covered and digested on the water-bath until the mass is thoroughly 
disintegrated. To hasten this, the supernatant liquid may after 
awhile be poured oft', the residue being washed into a small porcelain 
mortar, ground up, returned to the dish and boiled with fresh water 
until no hard grains are left. The total liquid is then filtered, and 
the residue is washed with hot water. The filtrate (which should 
amount to about 500 c.c.) is nearly neutralized with nitric acid (me¬ 
thyl orange being used as indicator), some pure sodium bicarbonate 
is at once added, and the solution (in a platinum dish, it one large 
enough is at disposal, otherwise in a beaker), is placed on the water- 
bath, when it speedily becomes turbid through separation of silica. 
As soon as the solution is warm it is removed from the bath, stirred, 
allowed to stand for two or three hours, and then filtered by means 
of the filter-pump and cold wash-water. 

The filtrate is concentrated to about 250 c.c., and nearly neutral¬ 
ized, as before; some sodium carbonate is added ; and the phosphoric 
acid is precipitated with silver nitrate in excess. The precipitate is 
filtered off and washed with hot water, and the excess of silver in 
the filtrate is removed with sodium chloride. The filtrate from the 
silver chloride (after addition of some sodium bicarbonate) is evap¬ 
orated to its crystallizing point, then cooled and diluted with cold 
water ; still more sodium bicarbonate is added, and the whole is 
allowed to stand, when additional silica will separate and is to be 
filtered off. 

This final solution is nearly neutralized, as before; a little sodium 
carbonate solution is added; it is heated to boiling, and an excess of 
solution of calcium chloride is added. The precipitate of calcium 
fluoride and carbonate must be boiled fora few minutes, when it can 
be easily filtered and washed with hot water. 'The washed precipi¬ 
tate is then washed from the filter into a small platinum dish and 
evaporated to dryness, while the filter, after being partially dried 
and used to wipe oft" any particles of the precipitate adhering to the 
dish in which it was formed, is burned, and the ash is added to the 
main precipitate. This, when dry, is ignited, and allowed to cool; 
dilute acetic acid is added in excess, and the whole is evaporated to 
dryness, being kept on the water-bath until all odor of acetic acid 
has disappeared. The residue is then treated with hot water, di- 


12 PHOSPHATE CHEMISTRY AS IT CONCERNS THE MINER. 

gested, filtered ou a small filter, washed with hot water, partially 
dried, put into a crucible, carefully ignited and weighed as CaFJ 2 . 
The CaFl 2 is then dissolved in H 2 S0 4 by gentle heating and agita¬ 
tion, evaporated to dryness on a radiator, ignited at full red heat and 
weighed as CaS0 4 . From this weight the equivalent weight of 
CaFJ 2 should be calculated, and should be very close to that actually 
found as above, but should never exceed it. The difference, which 
is generally about a milligramme (sometimes more), is due to silica 
precipitated with the fluoride. The percentage of fluorine is, there¬ 
fore, always calculated from the weight of the sulphate, and not from 
that of the original fluoride. 

The main improvements in this method are the use of sodium 
bicarbonate to separate the silica, and the keeping of the earlier so¬ 
lutions as dilute as possible, which cannot be done if ammonium 
carbonate is used for the separation of the silica. These changes 
make the fluorine estimation, although still tedious, far more rapid 
than before, and the results are very satisfactory. 

The following are the results obtained by these methods in the 
analysis of the last twelve samples examined. In these cases the 
methods, as given above, have been followed in all their details, and 
the figures are fairly representative : 


Sample No. 

Alfe 

2 

Pet cent. 

CaO 

Per cent. 

P2O5 

Per cent. 

FI. 

! 

Per cent. 

55 

‘2.30-2.28 

50.99-50.63 

35.37-35.33 

2.66-. 

111 

4.20-4.19 

47.01-46.99 

33.91-33.90 

2.35-2.24 

99 

2.50-2.62 

52.01-52.02 

38.78-38.79 

ti , — . , 

96 

4.23-4 22 

50.09-50.07 

38.82-38.86 

2.46 -2.45 

98 

7.93-7.79 

44.04-44.05 

35.28-35.10 

2.55-2.59 

1 

2.54-2.49 

33.10-33.03 

21.05-21.06 

3.17-. 

63 

.46- .48 

54.46-54.42 

37.70-37.77 

3.08-3.00 

940 

3.15-3.19 

46.06-46.00 

31.49-31.51 

1.87-1 86 

950 

2.20-2.16 

45 69-45.68 

28.49-28.45 

2.45-2.54 

947 

3.47-3.55 

42.74-42.81 

28.33-28.38 

2.48-2.41 

937a 

4.05-4.06 

47.96-47.94 

34.77-34.67 

2.72-2.73 

937b 

9.39-9.37 

15.95-15.89 

13.55-13.61 

.88-. 



























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